Removal of entrained air from cellulose pulp before bleaching of the pulp

ABSTRACT

Preliminary to being delivered by a &#39;&#39;&#39;&#39;thick stock pump&#39;&#39;&#39;&#39; means to a generally vertical bleaching vessel there is added to a cellulose pulp a fluid, such as steam, oxygen or a gaseous bleaching agent to flow countercurrent to the pulp in such manner that any air in the pulp is displaced and is replaced by bleaching agent before bleaching of the pulp in the bleaching vessel.

, [22] Filed:

United States Patent Engstrom [54] REMOVAL OF ENTRAINED AIR FROM CELLULOSE PULP BEFORE BLEACHING OF THE PULP [72] Inventor:

[73] Assignee:

Hans-Erik Engstrom, Sundsvall, Sweden Sunds Aktiebolag, Sundsvall, Sweden Apr. 1, 1971 [211 A p]. No.1 130,504

Related US. Application Data [63] Continuation of Ser. No. 771,593, Oct. 29, 1968,

abandoned.

[ 30] Foreign Application Priority Data Nov. 10, 1967 Sweden ..l5463/67 [52} U.S.Cl ..162/l9,8/1ll, 162/63, 162/65, 162/66, 162/68 51 1111. c1 ..D2lc 3/26 [58] r1616 61 Search ..l62/63, 65, 66, 68, 17,18, 162/19; 8/111, 156

1 51 June 6,1972

[56] References Cited UNITED STATES PATENTS 2,147,618 2/1939 Rawling 162/65 3,074,842 1/1963 Strong ..162/66 3,313,678 4/1967 Rydin 162/19 Primary Examiner-S. Leon Bashore Assistant ExaminerA1'thur L. Corbin Attorney-Pierce, Schefiler & Parker [57] ABSTRACT 5 Claims, 1 Drawing Figure REMOVAL OF ENTRAINED AIR FROM CELLULOSE PULP BEFORE BLEACHING OF THE PULP This application is a continuation of US. Pat. application Ser. No. 77 l ,593, filed Oct. 29, 1968, now abandoned.

According to the copending US. Pat. application Ser. No. 771,071 filed Oct. 28, 1968, by Rudi Schleinkofer, cellulose pulp is bleached with a gaseous bleaching agent, for example, oxygen, the bleaching agent being supplied to a downwardflow bleaching vessel, preferably a bleaching tower, into a space at the top of the tower above the column of pulp present in the vessel and the pulp falling down from the inlet opening of the tower being, within said space, subjected to a mechanical working, which has for its object partly to finely break up or disintegrate the pulp, partly to create a circulation of gas within said space, in order that the reaction between the pulp and the gaseous bleaching agent should be facilitated.

When using a gaseous bleaching agent the bleaching process should be carried out at a high pulp consistency, for example, at 20 percent consistency. Pulp of such a high consistency must, however, be transported to the inlet opening at the upper part of the bleaching tower and, the pressure in the tower at the pulp inlet opening preferentially being 8 l kiloponds/cm it is suitable to transport the pulp through a conduit to the inlet opening in question. The most suitable means for effecting such pulp transportation is a so-called thick stock pump. In connection with pulp transportation of the kind mentioned a great deal of air will accompany the pulp and, together with the latter, be introduced into the gas space at the top of the tower. ln bleaching towers, intended for bleaching by means of oxygen, the oxygen in the air supplied with the pulp will react with the pulp during the bleaching process, the nitrogen gas, however, being separated. The consequence is that nitrogen gas will accumulate within the tower, which may involve considerable drawbacks, because gas not taking part in the bleaching process is difficult to remove.

The present invention has for its object, by means of a special method and an apparatus for carrying out said method, to eliminate the above described drawbacks, at the same time providing for considerable advantages.

The new method in connection with the bleaching of cellulose pulp by means of a gaseous bleaching agent, for example, and preferably, by means of oxygen, in a downward-flow bleaching tower consists, fundamentally, therein that prior to feeding the pulp into a conveying means, for example, a thick stock pump, serving for transferring the pulp to the bleaching tower, a gaseous bleaching agent is supplied to the pulp in such a manner that air, contained in the pulp is wholly or partly, displaced and replaced by a gaseous bleaching agent, and that for carrying out the bleaching process an additional amount of gaseous bleaching agent, which may be required, is added to the pulp at some other place. This can preferably be done in a space at the top of the bleaching tower above the column of pulp in the tower.

An apparatus suitable for carrying out the method is characterized, fundamentally, by at least one conveying means for feeding cellulose pulp of, preferably, high consistency to a bleaching tower, to which the conveying means may be connected by means of at least one conduit, furthermore at least one conduit,.for supplying pulp'to said conveying means and at least one conduit for supplying a gaseous bleaching agent to the pulp near the conveying means. Then the supplying of the gaseous bleaching agent is meant to take place in such a manner that air, contained in the pulp, is gradually displaced and is, wholly or partly, replaced by the gaseous bleaching agent, which preferably is added in such an amount that part of it will leave together with the air displaced from the pulp. Preferably there is also at least one conduit for supplying bleaching agent at some other place.

In accordance with the invention it is suitable to add the gaseous bleaching agent to the pulp immediately before the pulp enters the conveying means and then to adapt the amount of bleaching agent and the pressure in such a manner that the gaseous bleaching agent will, at least to a little extent, be caused to flow counter the direction of feed of the pulp.

Preferably such large an amount of bleaching agent is added that part thereof will flow on through the conduit towards the place where the pulp is being supplied, possibly all the way up to said place and, alternatively, out of the opening of said conduit. Furthermore, the amount of gaseous bleaching agent, added before the conveying means, may be controlled automatically. The gaseous bleaching agent may be added partly before the means for conveying the pulp to the bleaching tower, partly at the upper end of the tower and, furthermore, the gaseous bleaching agent may be added at an essentially lower pressure before the conveying means for the pulp than in the bleaching tower itself.

Instead of, or together with, oxygen there may be used. for the displacement of the air in the pulp, some other gaseous bleaching agent and/or vapor, preferably steam. If, for example, steam is used then heating of the pulp is obtained at the same time.

The following is a description of a preferred embodiment of a plant for carrying out the new method, reference being had to the accompanying diagrammatic drawing which shows partly in vertical elevation and partly in section, a plant, according to the present invention.

Numeral 1 indicates a bleaching tower, which may be a con ventional downward-flow tower, provided with a supply conduit 2 for cellulose pulp, connected to the tower at the top thereof, and with an outlet conduit 3 for treated pulp, located at thebottom of the tower. At the bottom part of the tower there are, in a manner known per se, provided dilution nozzles 4 and propeller stirring means 5 for the purpose of providing a uniform decrease of the consistency of the pulp. The pulp is supplied to the tower by a conveying means, which, in the embodiment shown, is presumed to consist of a so-called thick stock pump 6, to which the pulp is led from a filter 7, via a screw conveyor 8, a conduit 9 and a conduit 10 connected to the thick stock pump. The conduits 9 and 10 may be replaced by one single conduit, which then preferably extends more straight to the thick stock pump 6. In the embodiment shown the conduit 10 has a larger diameter than the conduit 9, which is, in the first place, due to the fact that the conduit 10 encloses a screw conveyor 11. To that portion of the conduit 10, which is located next to the thick stock pump 6, which portion also may be characterized as a screw trough, there is connected a conduit 12 for a gaseous bleaching agent, for example, oxygen and/or vapor, preferably steam. The adding of a gaseous bleaching agentto the bleaching tower 1 also takes place through a conduit 13. The gaseous bleaching agent may be added at any point before dilution occurs. The bleaching tower usually being under a high pressure there is provided, on the upper portion of the tower, a conduit with a safety-valve I4. Numeral 15 indicates a movable, preferably rotatable, member adapted to work the pulp upon its entering the tower. The means in question fonns the object of the aforementioned co-pending US. Pat. application Ser. No. 771,071 filed Oct. 28, 1968 by Rudi Schleinkofer. For controlling the supply of gasand/or vapor (steam) to the conduit 12, which is connected to the conduit 10 just before the thick stock pump 6 there is provided a control valve 16, which preferably is adapted to be controlled by a feeler 17 for feeling the concentration of the gaseous bleaching agent and/or vapor (steam) which may ascend within the conduit 9. The feeler 17 preferably is located at a larger distance from the thick stock pump 6 than is the place of connection of the supply conduit 12in the neighborhood of said pump.

When using, for example, oxygen as a bleaching and air-displacing agent the plant operates in the following manner.

Dewatered cellulose pulp, having a consistency of say about 20 leaves the filter 7 in order to be moved sideways by the screw conveyor 8 to the substantially vertical, long conduit (tube) 9 to the nearest'portion of the conduit (screw trough) 10, where the pulp is caught by the-screw conveyor 11 and is fed on into the thick stock pump 6 in order to be moved on to the bleaching tower l. The pulp which is to be fed into the thick stock pump 6 by means of the screw conveyor 11 contains a great deal of air, which, wholly or partly, is displaced or replaced by oxygen arriving from the conduit 12. In that case some oxygen may ascend within the conduit 9 and the feeler 17 then gives an impulse for controlling this quantity of gas to a predetermined value. In order to guarantee a good displacement of the air within the pulp prior to its entering the thick stock pump a certain excess of oxygen may be allowed without any mentionable amount of oxygen leaving at the upper opening of the conduit 9. As a matter of fact the oxygen is absorbed, at least partially, by the falling pulp.

A plant according to the invention is not restricted to the embodiment as shown and described, but several modifications may be devised within the scope of the invention. For example, the supply conduit 12 for the gaseous medium need not be connected to the pulp supply conduit near the pulp conveying means 11 but it may, instead, have its connection located further away from the conveying means.

lclaim:

1. In a method for bleaching cellulose pulp with a gaseous bleaching agent in a down-flow bleaching tower having an upper space filled with a gaseous bleaching agent introduced under pressure and a column of pulp within the tower below said space, said pulp being introduced into the top of said tower against the pressure of the bleaching agent by a pump means between a first and second interconnected conveying conduit for the pulp, the improvement comprising passing pulp with entrained air into the inlet end of the first conveying conduit, passing the pulp toward the pump means, introducing a fluid selected from the group consisting of steam, oxygen and a gaseous bleaching agent into the pulp in the first conveying conduit before the pulp arrives at the pump means and adjacent thereto, said fluid being introduced under such pressure that the fluid flows counter to the pulp passing through the first conveying means to replace and expel at least a portion of the entrained air in the pulp, passing the pulp through the pump means, pumping the pulp through the second conveying conduit to the top of the bleaching tower and introducing a gaseous bleaching agent into the tower at a point before dilution occurs for bleaching the pulp.

2. In a method as claimed in claim 1 wherein the fluid is steam.

3. In a method as claimed in claim 1, wherein the fluid is oxygen.

4. In a method as claimed in claim 1 the further step comprising detecting the amount of fluid in the pulp at a point upstream from the point of introduction of said fluid and controlling the amount of fluid so introduced dependent on the value of the amount detected.

5. in a method as claimed in claim 1 wherein the gaseous bleaching agent introduced into the tower is introduced into the space at the top of the tower above the column of pulp therein. 

2. In a method as claimed in claim 1 wherein the fluid is steam.
 3. In a method as claimed in claim 1, wherein the fluid is oxygen.
 4. In a method as claimed in claim 1 the further step comprising detecting the amount of fluid in the pulp at a point upstream from the point of introduction of said fluid and controlling the amount of fluid so introduced dependent on the value of the amount detected.
 5. In a method as claimed in claim 1 wherein the gaseous bleaching agent introduced into the tower is introduced into the space at the top of the tower above the column of pulp therein. 